1. Field of the Invention
The embodiments discussed herein cover the fabrication and use of a free floating element in a micro electro mechanical system (MEMS) that is un-tethered during the release process. The embodiments can be used in a rocker design where the free floating element lands on a rocking post and has two stable positions, either leaning to the left or leaning to the right. Such a structure can be used as a bi-stable switch for optical applications. An array of these floating rockers can be used for video projectors. These projectors can be used in conjunction with a computer for presentations or for video entertainment or in high definition applications as a cinema projector. The same technology can also be used for back projection TV. Because it is not mechanically tethered like a cantilever or tethered rocker, the floating rocker is only held in any stable position by adhesion forces. The floating rocker can be switched using any force, from inertial, magnetic and electrostatic. The advantage of not having a spring like cantilever attachment means that the floating rocker can move between one stable state and the next very quickly as there is no spring like restoring force from the cantilever to overcome. This allows for faster dithering and giving a better image with less perceptible flicker and better control of color and grey scale. It is also better for games applications or sports programs. Not having the restoring spring reduces the cost of manufacture as fewer processing steps are utilized to fabricate the device and variation in the production of the spring arms is removed. Also, not having a restoring spring like arm, variations in temperature do not alter the switching properties, making the switching properties less susceptible to temperature variations. Because the mirror is now not attached, more of the area can be used for reflection as there is no need to connect the mirror to the substrate with a via as is required if it is tethered.
2. Description of the Related Art
One problem that is often encountered with the fabrication of MEMS devices is that when they are designed as cantilevers, there is a variation in the restoring force of the cantilever due to variations in the width, thickness and length as well as adhesion forces at the cantilever. The spring like arm which is normally used to hold a spring like rocker in position is removed in the case of the floating rocker. This spring like arm sets the voltage required for switching and is sensitive to variations in the processing parameters such as the lithographic width and length.
When the rocker touches down, variations in adhesion also come into play. In making a display, it is important that each pixel is closely spaced giving a high packing density. This means that there is not room to fit the restoring arm in the same metallization layer as the reflective pixel material. Also, the reflective pixel material has to be optimized for high reflectivity and so can not be made at the same time as the spring arm which needs to be thin to make it compliant.
The use of a free floating element has been discussed before in U.S. Pat. No. 6,441,405 (B1). That patent discussed the use of a free floating element and how it has advantages over a cantilever, but does not fully disclose how this can be used to make a bi-stable rocker display which will not switch due to variations in temperature.